ABSTRACT
Spemann (1931) pointed out that different parts of the organization centre in the amphibian gastrula have different properties, in that the more anterior regions tend, other things being equal, to induce the formation of more anterior parts of the neural system, such as the brain and associated sense organs, while the posterior parts tend to induce the neural tube of the trunk or tail. Living organizer grafts frequently become more or less fused with the induced tissues, so that if a definite organ is finally produced, the individuation of it involves both inducing and induced materials, the former playing the leading part. When dead pieces of the organizer are used as grafts, they cannot be similarly incorporated, and any individuation which may occur must have arisen entirely within the reacting material. It might still be possible, however, that the dead organizer was able to specify which region of the embryonic axis was represented by the induction, and thus guide the individuating processes. It has been known, however, since the work of Holtfreter (1934a) that organizer material after being killed loses its power to induce mesodermal tissues. It evocates only the appearance of neural tissue. If this becomes individuated into any recognizable organ, it is usually found to represent some region of the brain or eye, while the organs resulting from secondary inductive effects of the brain (e.g. nose or ear) are also frequently found. It is certainly not impossible to find a dead material which can induce more posterior regions of the axis, since Holtfreter (1934b), Chuang (1938) and Toivonen (1949, 1950) have demonstrated such activity on the part of dead adult tissues or extracts from them. No phenomena of the kind have, however, been described as resulting from the action of dead parts of the gastrula organizer, and it has been usual to consider that this material loses its trunk-inducing powers, and becomes solely a specific head-organizer, when killed.
For Pis. 20 and 21 see following article.
